1. Field of the Invention
The present invention relates to a pulse-width modulation (PWM) circuit and a switching amplifier using the PWM circuit.
2. Description of the Related Art
A conventional switching amplifier (e.g. an audio amplifier) is disclosed in JP-A-2003-179445, for example. The amplifier includes a pulse-width modulation circuit (PWM circuit) and a power amplifying circuit. The PWM circuit performs pulse-width modulation on inputted signals and outputs modulated signals to the power amplifying circuit, provided with a plurality of switches.
In such a switching amplifier, PWM signals are supplied to the power amplifying circuit from the PWM circuit for the purposes of controlling the voltage applied across the load (e.g. a loudspeaker). In the switching amplifier, the duration of the power voltage application to the load is varied. In this manner, the voltage applied to the load is virtually variable.
FIG. 20 is a block diagram showing a switching amplifier incorporating a conventional PWM circuit. According to this switching amplifier, as the user is adjusting the volume by operating the volume control knob (not shown) associated with a variable resistor 40, the amplitude of an audio signal (i.e. an input signal outputted from the audio signal source AU) is varied. Then, the audio signal is inputted to a PWM circuit 41, where the signal is modulated to a pulse signal having different time-widths corresponding to the amplitudes.
The output from the PWM circuit 41 is inputted to a power amplifying circuit 42. The power amplifying circuit 42 converts the input signal to a pulse signal having different levels of power source voltage corresponding to the level of amplitude. Then, the signal goes through a low pass filter LPF to remove high-frequency components, and the signal is outputted to the load L (e.g. a loudspeaker).
In the above switching amplifier, the amplitude of the audio signal is varied by the variable resistor 40, which is connected to the audio signal source AU. However, this method may suffer the following problem. Specifically, because the variable resistor 40 varies the audio signal amplitude directly, input impedance as viewed toward the audio signal source AU (signal source impedance) is also varied. The variation of the audio signal amplitude affects switching amplifier gain characteristics and S/N ratio. For example, while the noise level remains unchanged, the audio signal level is decreased. Unfavorably, this leads to a poor S/N ratio.